Abstract

The nakhlites are a group of mafic
igneous rocks that crystallized on Mars at ca. 1.3-1.4
Ga [1-5]. They are amongst the least shocked Martian
meteorites, with cumulate igneous textures (Fig. 1) and
thus provide a crucial record of igneous activity and
fluid-rock interaction on the red planet.
Crystallization of the nakhlites. Understanding the
original structure of the nakhlite source is crucial for
using these stones to explore Martian igneous processes.
In particular, are the different nakhlites from a single
thick and differentiated lava flow/sill [6; 7], or do
they instead represent magmatically related – but distinct
– flows/intrusions? If the various meteorites are
derived from separate units, then there will be differences
in crystallization ages within the nakhlite suite –
which may be identified if these differences are sufficiently
large relative to the attainable precision of radioisotopic
dating techniques. Some studies [e.g., 5] appear
to resolve age differences between different
stones, and we aim to test the single vs. multiple unit
hypotheses via application of detailed 40Ar/39Ar stepheating
of six nakhlites.

Alteration of the nakhlites, and the timing of waterrock
interaction. In addition to primary magmatic
minerals and glasses, the nakhlites contain secondary
minerals including clays and carbonates that were precipitated
by Martian aqueous fluids prior to impact
ejection (Fig. 2) [8-10]. When did this alteration occur?
As the clays contain potassium, they are amenable
to K-Ar and 40Ar/39Ar dating. K-Ar data from Lafayette
suggests the alteration phases formed between 0
to 670 Ma [11]. We consider further the timing of alteration
using our 40Ar/39Ar data.